With the tendency to decrease the weight of and apply a high stress to automobiles and the like, valve springs, clutch springs and the like that are used in engines, clutches and the like have been designed so as to apply higher stress. Therefore, the applied stress to the springs is increased, which requires those springs to have excellent fatigue properties and setting resistance, in particular, to have excellent fatigue properties due to being less likely to cause fatigue failure because of their internal defects.
In recent years, most of valve springs, clutch springs and the like have been manufactured by quenching and tempering, called oil tempering, a drawn wire rod to forma steel wire with a tempered martensite microstructure, and then spring-coiling the obtained steel wire at an ordinary temperature (cold working). Some of springs are produced by spring-coiling a drawn wire rod at an ordinary temperature, and then quenching and tempering the obtained spring-coiled wire. In either manufacturing method, the microstructure of the steel material of the spring is tempered martensite.
The above-mentioned tempered martensite is convenient to achieve high strength, and advantageously capable of enhancing the fatigue strength and setting resistance. However, the toughness and ductility of the steel material are reduced with increasing strength, which might easily cause a breakage due to internal defects in the steel material such as inclusions and the like. This could result in degradation of the fatigue properties.
The following have been proposed against the degradation in the fatigue properties by increasing strength due to the tempered martensite microstructure. For example, Patent Document 1 disclose a technique as follows: when Li is included in a total amount of Li of 0.020 ppm to 20 ppm in terms of mass, “Li is trapped in a complex oxide during manufacturing the steel to form a single-phase complex oxide (for example, CaO—Al2O3—SiO2—MnO—MgO—Li2O based complex oxide and the like). When heating this steel, material to a hot working temperature, the Li-containing complex oxide based inclusions are progressively separated into a glassy phase and a crystalline phase. The crystalline phases are finely precipitated as the equilibrium phase in the glassy single-phase inclusions. When blooming and hot-rolling the steel material in this state, a glassy portion demonstrates the excellent drawability because of its low melting point and low viscosity. Meanwhile, stress is concentrated on an interface between the crystalline phase and glassy phase during rolling, whereby dividing with remarkable ease, making the inclusions much finer”. Consequently, patent document 1 explains that the fatigue properties can be improved. However, this technique is not easy because control is needed during a steel manufacturing process in order to obtain a single-phase complex oxide. Furthermore, the technique is susceptible to external factors, including a heating condition, heat treatment temperature and the like during manufacturing.
Patent Document 2 discloses a steel wire for a spring obtained by patenting and drawing a steel material, followed by quenching and tempering. The patenting treatment involves heating the steel material at 900 to 1050° C. for 60 to 180 seconds for austenitizing, and then heating at 600 to 750° C. for 20 to 100 seconds under an isothermal transformation condition. The steel wire for a spring has a tempered martensite microstructure, and contains, in percent by mass, C: 0.50 to 0.75%, Si: 1.80 to 2.70%, Mn: 0.1 to 0.7%, Cr: 0.70 to 1.50%, and Co: 0.02 to 1.00%, and the balance consisting of Fe and inevitable impurities. The steel wire has a reduction of area after the quenching and tempering of 40% or more. In addition, the steel wire has a shear yield stress of 1,000 MPa or more after heat treatment at a temperature of 420° C. or higher and 480° C. or lower for 2 or more hours and then the quenching and tempering. That is, the technique specifies the patenting heat treatment, the reduction of area after the quenching and tempering, and the shear yield stress after a heat treatment which is equivalent to a nitriding treatment, thereby ensuring the fatigue properties and high toughness. However, the above-mentioned steel wire essentially includes Co and includes large amount of Cr, leading to high cost of the alloy.